I'm mainly looking at an oil pressure sensor at the moment.
I have temp sensors sorted by using a thermistor in a voltage divder circuit. Which is easy, I understand how this works.
But how do you get the reading from sensors with just one wire?
Obviously it grounds through the engine, and has the bias resistor inside the sensor.. But that leaves me with either voltage in, or voltage out of the single pin. Obviously I can't get voltage out without putting voltage in.
So how does it work? Seems like black magic to me..........
Most simple sensors work as a switch. The signal wire is grounded when the switch is closed and floating when the switch is open. Use a pull-up resistor to bias the signal wire to +5. The signal will read HIGH when the switch is open and LOW when the switch is closed. You can enable the built-n pull-up resistor on an input pin with:
Just to add to what John said, some do act as variable resistors to ground (think potentiometer controlled by heat, pressure or vacuum). If the vehicle has mechanical gauges, or can tell you the precise temperature then its probably an analog output sensor. Most modern sensors are completely isolated from the engine block ground by using an extra wire or two.
Back in the day, the "wire" from an oil pressure gauge wasn't a wire, it just looked like one.... it was a plastic pipe full oil under pressure which operated some mechanical gubbins in the gauge, and which sprayed boiling oil in the driver's compartment if something broke.